Modified nickel ferrite



United States Patent C MODIFIED NICKEL FERRITE Donald H. Baird, NewYork, Joseph John Dymon, Flushing, and Samuel Natansohn, Brooklyn, N.Y.,assignors,

by, mesne assignments, to Sylvania Electric Products Inc., Wilmington,Del., a corporation of Delaware No Drawing. Filed June 13, 1957, Ser.No. 665,591

1 Claim. (Cl. 252-625) Our invention is directed toward ferritematerials and more particularly relates to ferrite materials which findapplication at microwave frequencies.

It is known that when an electromagnetic wave propagated at microwavefrequencies is supplied to a circuit element formed from magnetizedferrite material, the permeability of this element does not have aconstant value but rather varies in accordance with variations in thewave polarization. This variable permeability is extremely useful inmicrowave applications.

.Ferrite materials of this type, when magnetized by a direct magneticfield, will absorb a large percentage of incident electromagnetic energywhen this energy falls Within a given narrow band of frequencies, thefrequency range being determined by the intensity of the magnetic field.The frequency band is defined as the ferromagnetic resonance absorptionband. I V

In conventional circuit applications, ferrite materials are used atfrequencies which exceed the frequencies falling within the absorptionband. Since the frequencies in the absorption band are uniquelyspecified by a given value of field intensity, in order for a ferriteelement to be operated at frequencies above those in the absorptionband, the field intensity must be reduced below this given value.

The field intensity, however, cannot be reduced indefinitely. When thefield intensity is reduced sufficiently to permit operation atrelatively low, microwave frequencies, the permeabilities ofconventional ferrites become erratic and unpredictable. Morespecifically, in conventional ferrites it is found that this erraticbehavior can be expected when the frequencies to be used expressed inmegocycles per second are less than approximately 2.8 times as large asthe saturation magnetization of the ferrite expressed in gauss. Hence,it is desirable to adjust the saturation magnetization in accordancewith the operating frequency desired.

In addition to the saturation magnetization, two other parameters offerrite materials become important in microwave applications. The firstof these, the Curie temperature, is a measure of the maximum temperatureuse to which ferrite materials can be subjected without losing theirmagnetic properties and hence becoming un-' usable. Since thesematerials necessarily absorb energy during operation and are therebyheated, the Curie temperature limits the power handling capacity ofdevices incorporating such materials. The second parameter, thedielectric loss, is a measure of energy absorbed by a ferrite duringoperation and, to reduce insertion losses and increase efiiciency indevices utilizing such material, must be as small as possible.

We have invented a new type of ferrite material which has a lowdielectric loss, a relatively high Curie temperature, and a saturationmagnetization which can be varied at will over a wide range.

Accordingly it is an object of our invention to provide new and improvedferrite materials characterized by a low dielectric loss, a relativelyhigh Curie temperature, and a saturation magnetization variable over awide range.

Another object is to improve ferrite materials operable at radio ormicrowave frequencies by increasing the Curie temperature and decreasingthe dielectric loss of such materials at these frequencies.

Still another object is to provide new and improved ferrite materialsoperable over a wide frequency range and having a saturationmagnetization which can be varied in accordance with any chosenfrequency within said range.

These and other objects of our invention will either be explained orwill become apparent hereinafter.

In accordance with the principles of our invention, the oxides ofmanganese, nickel, iron and aluminum are reacted together in the solidstate to produce our ferrite material. The proportional concentration ofeach oxide in the total mixture, as expressed in molecular ratios, isdefined by the approximate formula z s-l-( 2 3 wherein (a) rangesbetween 0.02-0.10, and the quantity (b-i-c) ranges between 0.95-0.99.The ratio c/b can in principle have any value, but our best results havebeen obtained when the ratio c/b falls within the approximate range 0-1.

We have found that depending upon the particular composition selected,the saturation magnetization can be varied at will within theapproximate range -2700 gauss with a corresponding range of Curietemperature from 275 to 575 C. Further, the dielectric loss as expressedin terms of the dielectric loss tangent at a frequency of 10 megacyclesper second (the dielectric loss tangent is a measure of the conductivityof a ferrite material when used as a dielectric in a capacitor) can beas low as 0.002 for compositions covering this entire range.

In the copending patent application of D. H. Baird and J. J. Dymon,filed January 3, 1956, Serial No. 557,- 429, there is disclosed aprocess for producing ferrites in which suitable compounds of themetallic elements are first mixed together in the proper proportions andfired in air at a temperature of about 950 C. The mixture is thensubjected to one or more calcining operations with intermediatepulverizing steps if required. The material is then pulverized once morewith addition of suitable binding or plasticizing agents and is pressuremolded into a compact. The end product is then produced by firing in airat a temperature between 1350 C. and 1500" C. for a period of from 6 to10 hours and then cooling this compact.

Our materials can be produced in accordance with the process taught inthe above identified application or alternatively can be produced inaccordance with more conventional ceramic processing techniques.

Our invention will now be described in detail with reference to theillustrative example which follows.

EXAMPLE Various ferrite materials satisfying the formula were preparedunder such conditions that (a) was held constant at .06 and the quantity(b+c) was held constant at 0.970. The saturation magnetization, Curietemperature, and dielectric loss tangent (measured at a frequency of 10megacycles per second) were then determined for each formation asindicated in Table I below.

As indicated by the above data, when quantities (a) and (b-l-c) are heldconstant and the ratio g is the values of the saturation magneizationand Curie temperature of the material approach the corresponding valuesof nickel ferrite (which has a saturation magnetization of approximately3000 gauss and a Curie temperature of approximately 580 C.) As the ratioc/ b is increased, the saturation magnetization decreases rapidly, andthe Curie temperature decreases at a relatively slow rate. Since A1 0and 'Fe O will always form a solid solution, irrespective of therelative proportions of each component, the ratio c/b can take any valuebetween Zero and infinity. However, our best results have been obtainedwhen the ratio c/b does not exceed 1.

When the quantity (b+c) is increased from .97 toward a maximum of about.99, and the ratio the values of the saturation magetization and Curietemperature of the material approach more closely the correspondingvalues of nickel ferrite. When this quantity (b-i-c) is decreased towarda minimum of about .95, the difierence between the values of saturationmagnetization and the Curie temperature of the material and thecorresponding values of nickel ferrite become more accentuated.

The manganese oxide content (a) determines, at least in part, thedielectric loss tangent of the material. When no manganese oxide ispresent for example, the dielectric loss tangent of the resultingferrite can exceed 1; such material will have a very high insertion lossandvwill be extremely ineificient. In order to reduce the dielectricloss tangent to an acceptable low value, (a) should have a minimum valueof about 0.02. On the other, hand,

as (a) is increased, the loss tangent tends to decrease. However, anexcess of manganese oxide will adversely influence the magneticproperties of the ferrite, such as the Curie temperature, so that (a)should have a maximum value of about 0.10. Since the valencies of themanganese in this product cannot be accurately evaluated at present, itwill be understood that the manganese in the product is not necessarilyin the divalent state.

While we-have shown and pointed out our invention as applied above, itwill be apparent to those skilled in the art that many modifications canbe made Within the scope and sphere of our invention as defined in theclaim which follows.

What is claimed is:

A ferrite material made by firing the oxides NiO, MnO, Fe O and A1 0 inair at a temprature of about 950 C., mixing the product with a bindingagent and sintering in air at a temperature between 1350" C. and 1500 C.for a period of 6 to 10 hours, said original components of NiO, MnO, FeO and A1 0 being defined by the approximate formula (1) NiO-l-(a)MnO-i-(b) Fe2 3+(C) A1 0 wherein the quantities (l), (a), (b), and (c)are mole ratios, the quantity (a) being equal to 0.06, the quantity,(b-l-c) being equal to 0.97, and the ratio c/b falling within theapproximate range 0084-0482, the dielectric loss tangent of said ferritematerial falling within the range 0001-0008, the Curie temperaturefalling within the range 285 C.-530 C., and the saturation magnetizationfalling within the range -2000 gauss.

References Cited in the file of thispatent UNITED STATES PATENTS2,179,810 Brill "Nov. 14, 1939 2,565,861 Leverenz et al. Aug. 28, 1951FOREIGN PATENTS 82,278 Netherland Aug. 5, 1956 1,117,385 France Feb. 20,1956 1,122,258 France May 22, 1956 OTHER REFERENCES

